1. Field of the Invention
The present invention relates to a power amplifier, and more particularly to a high output power amplifier with adjustable voltage transformation ratio of a transformer which is installed to output leads thereof, which can increase dynamic range and efficiency thereof as a voltage transformation ratio of the transformer is adjusted and load resistance is adjusted so as not to generate signal distortion phenomenon and not to decrease its efficiency.
2. Description of the Related Art
Generally, saturation power showing output power Pout of an amplifier can be expressed as following equation (1).Pout=Vsuppy2/Rload  (1)
Here, Pout, Vsuppy and Rload denote output power, source voltage and load resistance.
As shown in equation (1), when Vsuppy is adjusted, the output power Pout is controlled.
With reference to FIG. 1 of a circuit diagram illustrating a prior art high output power amplifier, a source voltage controller 113 performs conversion of input voltage 112 to adjust source voltage 114 inputted to the amplifier 115, using a DC-DC converter or a low dropout voltage regulator (LDO), for example.
However, since the source voltage controller 113 has limited control of the range of output power Pout 118, as shown in FIG. 1, as the load resistance Rload 119 is changed, the dynamic range of the output power Pout 118 can be more highly controlled.
Therefore, as load resistance 116 with respect to output of the amplifier 115 is adjusted using a variable load unit 117, the dynamic range and efficiency of the high output power amplifier of the prior art can be increased.
FIG. 2a to FIG. 2c are graphs describing changes of efficiency of a prior art high output power amplifier as load resistance is changed.
More specifically, FIG. 2a is a graph illustrating a voltage, current, and consumption power (a rectangular area which is indicated by slant lines) at the maximum output. FIG. 2b is a graph in a state wherein the efficiency of the high output power amplifier is decreased, compared at the maximum output power, because the consumption power is not reduced, when the output power is decreased to a half thereof, if the load resistance is not change. FIG. 2b is a graph in a state wherein the efficiency of the high output power amplifier is maintained, because the consumption power (a rectangular area which is indicated by slant lines) is decreased, when the load resistance is doubled, if the output power is reduced to a half thereof.
Therefore, with reference to FIG. 3a and FIG. 3b showing circuits for changing load resistance of a prior art high output power amplifier, a method for changing the load resistance of the amplifier is described as follows.
As shown in FIG. 3a, the load resistance is changed using varactor diodes 127 and 130. Also, the load resistance is changed by switching transistors 134 and 135, as shown in FIG. 3b. 
Here, the method using the varactor diodes 127 and 130 controls the load resistance such that, when voltage of the varactor diodes 127 and 130 is varied through control terminals 128 and 31, capacitance of the varactor diodes 127 and 130 is changed.
However, such a method has a disadvantage in that output voltage is clamped by turn-on voltage (approximately 0.7V) of the varactor diodes 127 and 130 and thus output power waveform is distorted.
In FIG. 3a, reference number 120 denotes a high output power transistor, reference number 121 denotes an input lead, reference numbers 122, 126, and 129 denote inductors, reference number 123 denotes a voltage source, reference numbers 124 and 125 denote capacitors, reference number 132 denotes an output lead, and reference number 133 denotes a load resister.
Also, as shown in FIG. 3b, the load resistance can be changed as switching transistors 134 and 135 are turned on/off.
However, the method using the switching transistors has drawbacks in that efficiency is relatively small since size of the switching transistor is relatively large and turn-on resistance of the switching transistors exsists.